There is provided a method for producing cycloolefin where the nickel which elutes in the liquid-contact portion of the reactor is removed from the reaction system of the partial hydrogenation reaction and cycloolefin can be stably produced from a monocyclic aromatic hydrocarbon over a long term. A method for producing a cycloolefin by a partial hydrogenation reaction of a monocyclic aromatic hydrocarbon in an aqueous phase, which contains a metal salt-containing acidic aqueous solution and a ruthenium catalyst in a reactor with a liquid-contact portion formed of a nickel-containing material, the method comprising a first step of bringing at least a part of the ruthenium catalyst contained in the aqueous phase into contact with oxygen, a second step of separating the aqueous phase containing the ruthenium catalyst that has been processed in the first step into a first phase containing the ruthenium catalyst at a high ratio and a second phase containing the ruthenium catalyst at a lower ratio than the first phase, and a third step of feeding the first phase to a reaction system of a partial hydrogenation reaction.